Greetings,

I have a 1991 350 SD with a climate control blower fan that seems to work when it wants. Everything is fine with the A/C and the vacuum operated mechanics. Sometimes the blower fan is completely dead and will not activate on any speed button. Then without warning, everything works. Any ideas where to troubleshoot? Seems electrical connection related?

There's also the posibility that the brushes in the fan motor are very worn down, and only make contact sometimes, or if they've been bumped around while driving to get them going etc....I'd check that. The regulator relay thing can also be a culprit.....but I'd start by checking wear on the fan motor/brushes, my old one was in terrible shape and actually started to melt ....the brushes and housing was so worn out....it lasted about 22 years though, so I suppose it filled its duty.

__________________
-diesel is not just a fuel, its a way of life-
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
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One possibility is a problem with the solder joints in the push button unit. If you remove the unit and take the cover off the bottom, you will see the joints at the intersection of the horizontal circuit board and the vertical boards on each end. Cracks in the joints can cause intermittent continuity. Reflowing the joints usually solves the problem. There are also relays in the pushbutton unit which can operate intermittently.
When the fan is not working, check to see if you get power at the fan connector. If you don't, the fan motor is not likely the problem..

Thanks guys for the suggestions but I have some more information. The climate controls and all fan speeds work just fine when the key switch is in the "on" position. Or if the climate control is engaged BEFORE starting the motor, everything is fine with the car running except when the system is switched off. Then nothing comes back on with the car running. Jiggling the key switch with the car running hasn't made a difference.

Is it possible a key switch going bad is the source of the problem? If so, is replacing the key/tumbler unit the solution or does the ignition switch with the contacts also need to be replaced?

__________________
2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

The problem is not likely the tumbler.
Also not likely the ignition switch.
With the engine runing, you turn on the a/c but the fan does not come on. Does the a/c compressor engage?

Yes, the A/C compressor engages and the air barely circulates cold at a below economy speed. I just have not figured out why I would have a problem with the fan speeds not working when the car was running.

Another strange thing happened this past weekend that may be related somehow to my thought about the ignition switch. I was walking through a store parking lot to my car and hearing a loud radio playing only to find out it was my radio playing with the ignition off (I never power off the radio when I shut off the car). When I put the key in the ignition to start the car, the radio shut off but came back on when the car was started.

__________________
2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

I don't have the wiring diagrams for your model. Earlier vehicles with ACC, however, had a cold engine cut-out switch which prevented the fan from working until the engine warmed up. (The switch was closed when the coolant temp reached around 40 degrees C.) The switch was intended to only work in the heater mode, but a bad relay in the push button unit could cause it to be a factor in other ACC modes. If you car has such a switch, you might try grounding the wire when the fan is not running (but should be) and see if it has any affect.
But that still does not explain why the fan would run with the engine off but not with the engine running. I would also try this:
Start the engine.
Select any ACC mode except off.
Fan does not come on.
Shut down engine with manual lever; do not move key from run position.
Observe whether the fan comes on when the engine shuts down.

THANKS, I'll try that tomorrow and play with it some more as well. I am new to the MB diesel world which I am finding is much more interesting than the previous two MB gassers I've owned.

__________________
2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

I had a chance today to test the following sequence:

Started the engine.
Selected any ACC mode except off.
Fan did not come on.
Shut down engine with manual lever; did not move the key from run position.
The fan DOES come on when the engine shuts down.

What does this correct diagnosis tell me?

__________________
2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

I tried the same test again today with the same following results:

Started the engine.
Selected any ACC mode except off.
Fan did not come on.
Shut down engine with manual lever; did not move the key from run position.
The fan DOES come on when the engine shuts down.

What does this correct diagnosis tell me? Is there a relay of sorts that may be faulty?

__________________
2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

That would suggest to me that the ignition switch is not the problem.

Agreed.

It could be the fan motor I suppose, but from the beginning it seemed more like an electrical connection type of problem. As you suggested earlier, I'll need to confirm power to the fan next and then work my way back to the pushbutton unit and any relays.

__________________
2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

Hello!

This might also help (or might not ) - I had a very similar problem in regards with the fan operation symptoms. After starting the engine the fan would work but it would quit after about 15 minutes every time. For no apparent reason. Wiggling buttons and a million of other things would not work. Especially the 15 minutes period when it would quit every time caused me headache.

I found it was the fan fuse located in the engine compartment. It wasn't melted, it was cracked in the middle of the strip. A microscopic crack barely to notice. When cold there would be a circuit but after some time when the fuse warmed up the circuit would break.

When your fan quits again try to jump the fuse terminals to exclude this possibility. Good luck

Today I started by removing and cleaning the fan fuse in the engine compartment. Everything looked good.

Next, I uncovered the fan motor and watched its operation. The fan always runs at the "circulation" air speed when the car is running with the A/C on but will not run faster by engaging any of the fan speed buttons. The fan will run at the economy, auto and max speeds but only if the engine is stopped and the the button is pre-set before turning the key switch on. So I think the fan motor is probably good.

Next, I removed the push button control unit and the bottom cover. The circuit board on this unit is a one piece board and I was unable to remove it but also saw no opportunity to solder any joints. I removed each connector pin confirming the solder to the wire was good and the connection block was good left and right.

The problem with the lack of fan speeds remains with nothing obvious so far to condemn so I am thinking I need to either replace the control unit or investigate further if anyone has any other ideas?

__________________
2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD